To examine the superior efficacy of mycophenolate mofetil compared with placebo in patients with rheumatoid arthritis. Mycophenolate mofetil (RS 61443) (MMF) is an immunosuppressant that reversibly inhibits the proliferative response of human peripheral mononuclear cells to T and B cell mitogens in vitro, and immunoglobulin formation by human B cells in response to polyclonal stimulators.. Thes properties make MMF a good candidate for use in patients with rheumatoid arthritis (RA). Phase I and II data indicatted that optimum effect and safety were achieved at a MMF dose of 1 g BID in patients with RA. The study design is a randomized, double-blind, placebo-controlled trial of MMF therapy in RA for 6 months, evaluating efficacy and safety, with an option for 6 months open label. Twelve patients are anticipated at Massachusetts General Hospital.